There's no 'operational' information in the FSM for 1998 either
Too bad, there's lots of nice buttons on that HU. The RDS feature in itself can be complex in most vehicles -- I'm thinking it's features are addressed using a combination of buttons. You should be in FM mode while using the button.
I don't have the 1998 Owners Manual, for sure it's detailed in there.
Sorry
I'll have a look at the multi-year/model DVD later, maybe its in there.